This invention relates to implantable rate responsive cardiac pacemaker systems and methods for rate control of cardiac pacing and, more particularly, rate responsive pacemaker systems that have the capability of adjusting the pacing rate as a function of QT interval.
Implantable rate responsive pacemakers and other implantable medical devices that include rate responsive pacing have been in existence for some years. In a normal patient who undertakes exercise or becomes involved in a stressful situation that calls for an increased heart rate, the patient""s normal feedback mechanisms provide for the desired increased rate. However, for a variety of reasons the patient""s normal feedback mechanisms may be impaired. For a single chamber ventricular pacing system, or if the intrinsic atrial rate is unreliable, the pacemaker must carry out the task of determining the physiologically desired rate. Rate responsive pacing systems have the aim of providing a substantially physiologic pacing rate by sensing one or more patient parameters and correlating desired rate with such parameters. Rate responsive pacemakers are available as single chamber or dual chamber pacemakers.
A variety of sensors for determining a desired pacing rate are known in the pacing art. The assignee of this invention has utilized the QT interval (xe2x80x9cQTxe2x80x9d) as a parameter that is responsive to patient demand. The QT interval is the time between contraction of the heart and the relaxation of the heart. This is manifested by electrical signals which are measured by the implantable device when the depolarization/repolarization waves pass the electrode system implanted within the patient""s heart. The QT interval can be thought of as having a rate dependent component and a stress dependent component. The rate dependent component causes the QT interval to shorten at higher rates as the heart pumps more often and therefore reduces the time it stays contracted. Conversely, the QT interval lengthens at lower rates. The stress level, either physical or mental, also causes a QT interval shortening at times of higher stress. Thus, when QT interval is measured at any given rate, its value can be based both on the rate dependent component and the stress component. In healthy hearts the QT interval will always correspond to the desired heart rate, and thus can be used as a sensor for the metabolic need of the patient.
The QT sensor, to be effective, must have knowledge of the expected QT interval corresponding to any given rate. This is generally accomplished by maintaining within the pacemaker a QT reference curve, also referred to as a QT (RR) curve. If the QT interval at a given rate is shorter than the corresponding reference curve value at that rate, this indicates that the pacemaker should start pacing at a higher rate. In prior art QT sensor systems, the QT interval has been measured only after a ventricular pace, for the reason that it is difficult to sense the T wave following an intrinsic QRS signal. In pacing, the depolarization wave, or QRS complex, starts at the lead electrode or electrodes where the stimulus is delivered, such that the xe2x80x9cQxe2x80x9d time is actually the stimulus time. Further, the Twave that results from a stimulus delivered through an electrode positioned adjacent the lead tip is reliably detected. However, in the case of an intrinsic QRST complex, the Twave comes from above the lead position, and can travel different routes, is less well defined and more difficult to sense. For this reason, QT rate responsive pacemakers have relied upon obtaining QT measurements in response to delivered pacing pulses. These are actually Stim-T values, measured from the time of delivery of the stimulus pulse. In such a system, when the patient""s heart is naturally providing intrinsic QRST complexes, it is not possible to update or adjust the QT (RR) reference curve. In such circumstances, the QT rate responsive pacemaker historically has periodically initiated a pacing overdrive routine, to capture the heart with higher rate pacing so as to obtain QT (RR) reference data. However, the overdrive feature is not desirable, as it paces the patient at a rate that isn""t called for. Further, it is not able to provide reference data throughout the normal range of the patient""s intrinsic cardiac activity.
There are several problems with obtaining QT interval from intrinsic beats. As noted, it is more difficult to detect the intrinsic T wave, which is not as well defined and has a smaller amplitude than does the T wave that follows an evoked QRS. The QT interval in the sensed case is shorter than that in the paced case. Further, the intrinsic ventricular contraction might originate from an ectopic focus, which in turn can cause a deviation from the QT interval anticipated when the ventricular contraction originates from the AV node. These problems must be overcome in any arrangement for sensing intrinsic QT intervals.
Examples of pacing systems incorporating a QT rate responsive pacing, as well as techniques for adjusting the QT reference curve, are listed in Table 1 below. Also listed are patents showing the use of DSP technology for determination and classification of a sensed cardiac event.
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
It is an object of this invention to provide an improved rate responsive pacing system that utilizes QT as the sensor parameter for determining desired pacing rate. It is a further object this invention to provide such a system and method of pacing which obtains values of both intrinsic and evoked QT, and constructs a QT (RR) reference curve across the full range of the patient activity. A further object is to provide a pacing system and method of rate control that utilizes a QT sensor and does not need to resort to overdrive pacing in order to get QT information.
In accordance with the above objects, there is provided a rate responsive pacemaker and system of controlling pacing rate that utilizes QT interval data obtained from intrinsic heartbeats. The system and method of this invention accomplish the above object by utilizing DSP or equivalent circuit technology in combination with software to recognize intrinsic Twaves and to determine QT intervals continuously whether the patient is being paced or whether the patient has an intrinsic heartbeat. A preferred algorithm for reliably detecting the intrinsic T wave provides for integration of the sensed signal for a predetermined time window following the QRS complex, and determines the time of a detected T wave by timing either the maximum of the T wave slope or the maximum amplitude of the T wave. QT data obtained during pacing is compensated to take into the account the difference between the stimulus-T wave time following the delivery of the pacing stimulus, and the intrinsic QT time interval.
It is a further object of this invention to recognize any ventricular contraction that is an ectopic focus, and to discriminate and not use such sensed ectopic beats for adjustment of the QT reference curve. This is accomplished by recognition of the different profile, or shape of the QRS complex of an ectopic beat, as compared to the normal depolarization wave. DSP technology is suitably used for event classification and identification, in order to discriminate ectopic beats.
The system and method of this invention can be utilized as well in a pacemaker or implanted cardiac devise that utilizes more than one sensor for determining a desired pacing rate. For example, U.S. Pat. No. 5,065,759 illustrates a dual sensor pacemaker which utilizes both QT and activity sensing. As stated in that patent, which is listed above in Table 1, the QT sensor supplements an activity sensor, and can provide important information when the patient is undergoing stress without being characterized by a level of activity that would indicate the desired pacing rate.